A device for detecting the deterioration of a catalytic converter for an engine having two or more cylinder groups, and a catalytic converter disposed in a common exhaust passage with which exhaust passages individually connected to the respective cylinder groups are merged, wherein the air-fuel ratio of the respective cylinder groups are controlled independently based on the output of the upstream air-fuel ratio sensors disposed in the respective individual exhaust passages. Since the air-fuel ratios of the respective cylinder groups are controlled independently, the phases of the air-fuel ratio oscillations of exhaust gases flowing into the common exhaust passage from the individual exhaust passages are usually not synchronized. However, during the engine operation, the phases of the air-fuel ratio oscillations of the exhaust gases are occasionally spontaneously synchronized due to slight differences of the periods of the cycle of the air-fuel ratio feedback controls in the respective cylinder groups. The device detects such a spontaneous synchronization, and determines whether or not the catalytic converter has deteriorated based on the output of the downstream air-fuel ratio sensor disposed in the common exhaust passage downstream of the catalytic converter.